Silver nanoparticles (AgNPs) and other metal nanoparticles such as copper, and palladium have attracted considerable interest in many applications owning to their intrinsic size- and shape-dependent effects on antibacterial, catalytic, electronic, and optical properties. Sigma-Aldrich Inc. provides hydrophobic silver nanoparticles dispersed in hexane with two particle size distributions of 3-7 nm and 5-15 nm, respectively. The prices of both silver colloidals are about US$180 for a quantity of 25 ml. As considering the expensive prices, researchers are eager to develop a more economical technique to prepare the dispersion of hydrophobic metal nanoparticles, such as silver and palladium, in organic solvents.
The direct synthesis of silver organosol (silver nanoparticles dispersed in an organic solvent) is known to be a problem due to the poor solubility of traditional water-soluble silver salts such as silver nitrate (AgNO3), silver sulfate (Ag2SO4), silver oxide (Ag2O), and silver halides (AgX, X═F, Cl, Br, or I) in organic media. Similarly, the preparation of other metal nanoparticles including copper, palladium, gold, and platinum are also limited in their poor solubility of the corresponding metal salts in organic solvents. Consequently, the two-phase based methods were developed and adopted to prepare metal organosol by using phase transfer agents. Sarathy, K. V.; Raina, G.; Yadav, R. T.; Kulkarni, G. U.; Rao, C. N. R. in an article entitled “Thiol-derivatized nanocrystalline arrays of gold, silver, and platinum” J. Phys. Chem. B 1997, 101, 9876 describe the procedures involving the transfer of metal hydrosols (metal nanoparticles dispersed in water) into an organic solvent by using concentrated HCl as the phase transfer agent. Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. in an article entitled “Synthesis of thiol-derivatized gold nanoparticles in a 2-phase liquid-liquid system” J. Chem. Soc. Chem. Commun. 1994, 801 report the synthesis of gold nanoparticles by reducing the AuCl− ions with sodium borohydride in the water-toluene phase system. In their report, AuCl− ions was transferred from water phase into toluene phase using tetraoctylammonium bromide as the phase transfer agent. The disadvantages of aforementioned two-phase method were the necessity of toxic phase transfer agents such as concentrated HCl or tetraoctylammonium bromide, as well as the complicated synthetic process.
It is well-known that silver carboxylates with long alky chain are utilized as the silver source in the field of thermographic and photothermographic imaging techniques. In addition, the solventless approaches have been adopted to produce hydrophobic AgNPs through thermal decomposition of silver carboxylates with long alky chain under high temperature condition. For example, Abe, K.; Hanada, T.; Yoshida, Y.; Tanigaki, N.; Takiguchi, H.; Nagasawa, H.; Nakamoto, M.; Yamaguchi, T.; Yase, K. in an article entitled “Two-dimensional array of silver nanoparticles” employed the thermolysis of silver carboxylates with long alky chain (e.g., those containing 14-18 carbon atoms) to fabricate the hydrophobic silver nanoparticles at 250° C. in N2 atmosphere. Similar approaches are also described in Lee, S. J.; Han, S. W.; Choi, H. J.; Kim, K. J. Phys. Chem. B 2002, 106, 2892 and in Yang, N. J.; Aoki, K.; Nagasawa, H. J. Phys. Chem. B 2004, 108, 15027. However, the forementioned silver salts of carboxylic acid with long alky chains were not suitable to serve as the silver ion sources in wet chemical synthetic process due to their low solubility in water and organic solvents. Jacobson, C. A.; Holmes, A. in an article entitled “Solubility data for various salts of lauric, myristic, palmitic, and stearic acids” J. Biol. Chem. 1916, 25, 29 showed the results of low solubility of silver laurate, myristate, palmitate, and stearate in various solvents including water, alcohols, and ether. In addition, Malik, W. U.; Jain, A. K.; Jhamb, O. P. in an article entitled “Solutions of soaps in organic solvents” J. Chem. Soc. A 1971, 1514 investigated the low solubility of silver salts of carboxylic acid in various organic solvents. Therefore, the lack of organo-soluble metal precursors is the main impediment to the preparation of hydrophobic metal nanoparticles in wet chemical synthetic process. From this viewpoint, finding organo-soluble metal precursors is a key to provide the opportunity to synthesis metal nanoparticles directly in organic media.
The nontoxic, non-flammable, inexpensive, and abundant nature of carbon dioxide (CO2) has attracted great attention as an ideal processing medium in the fields of material science and nanotechnology (Eckert, C. A.; Knutson, B. L.; Debenedetti, P. G. Nature 1996, 383, 313; Holmes, J. D.; Lyons, D. M.; Ziegler, K. J. Chem.-Eur J. 2003, 9, 2144; Johnston, K. P.; Shah, P. S. Science 2004, 303, 482; Shah, P. S.; Hanrath, T.; Johnston, K. P.; Korgel, B. A. J. Phys. Chem. B 2004, 108, 9574). Metal nanoparticles, including silver and gold nanoparticles, have been synthesized through supercritical CO2 (sc-CO2) technologies such as the water-in-CO2 (w/c) microemulsions (Ji, M.; Chen, X. Y.; Wai, C. M.; Fulton, J. L. J. Am. Chem. Soc. 1999, 121, 2631; Ohde, H.; Hunt, F.; Wai, C. M. Chem. Mat. 2001, 13, 4130; McLeod, M. C.; McHenry, R. S.; Beckman, E. J.; Roberts, C. B. J. Phys. Chem. B 2003, 107, 2693), rapid expansion of supercritical solution into a liquid solvent (RESOLD) (Sun, Y. P.; Atorngitjawat, P.; Meziani, M. J. Langmuir 2001, 17, 5707; Meziani, M. J.; Pathak, P.; Beacham, F.; Allard, L. F.; Sun, Y. P. J. Supercrit. Fluids 2005, 34, 91), sc-CO2 flow process (McLeod, M. C.; Gale, W. F.; Roberts, C. B. Langmuir 2004, 20, 7078), arrested precipitation (Shah, P. S.; Husain, S.; Johnston, K. P.; Korgel, B. A. J. Phys. Chem. B 2001, 105, 9433; Shah, P. S.; Husain, S.; Johnston, K. P.; Korgel, B. A. J. Phys. Chem. B 2002, 106, 12178.), and other specific approaches (Fan, X.; McLeod, M. C.; Enick, R. M.; Roberts, C. B. Ind. Eng. Chem. Res. 2006, 45, 3343; Moisan, S.; Martinez, V.; Weisbecker, P.; Cansell, F.; Mecking, S.; Aymonier, C. J. Am. Chem. Soc. 2007, 129, 10602; Esumi, K.; Sarashina, S.; Yoshimura, T. Langmuir 2004, 20, 5189). However, sc-CO2 is a poor solvent for many high molecular weight and polar compounds due to the low dielectric constant and polarizability per volume of CO2. Accordingly, CO2-philic fluorinated molecules including surfactants, capping ligands, and metal precursors are required in the prior art to enhance the solubility of compounds in sc-CO2, although they are economically and environmentally unfavorable. Besides, a quite high process pressure (generally over 100 bar) is required to dissolve adequate amount of the fluorinated reagents in compressed CO2.
Recently, a non-fluorinated agent having branched alky chains, isostearic acid (2,2,4,8,10,10-Hexamethylundecane-5-carboxylic acid), is successfully employed to disperse silver nanoparticles in scCO2. Bell, P. W.; Amand, M., Fan, X.; Enick, R. M.; Roberts, C. B. in an article entitled “Stable dispersions of silver nanoparticles in carbon dioxide with fluorine-free ligands” Langmuir 2005, 21, 11608 disclose the successful dispersion of isostearic acid-capped silver nanoparticles in high pressure (276 bar) CO2 or in CO2 with 10 vol % hexane as cosolvent. In their article, silver nanoparticles were synthesized within the cores of AOT reverse micelles in advance. AOT (sodium bis(2-ethylhexyl) sulfocuccinate) was utilized as the surfactant to form micelles with nanosized reaction space in aqueous medium. While silver nanoparticles were formed in the reverse micelles, isostearic acid was added to replace the AOT as the capping agent on the surface of the silver nanoparticles. Then, the authors successfully dispersed the isostearic acid-capped silver nanoparticles in compressed CO2. Anand, M.; Bell, P. W.; Fan, X.; Enick, R. M.; Roberts, C. B. in an article entitled “Synthesis and steric stabilization of silver nanoparticles in neat carbon dioxide solvent using fluorine-free compounds” J. Phys. Chem. B 2006, 110, 14693 disclose the in-situ synthesis of silver nanoparticles in the presence of isostearic acid in compressed CO2 wherein silver bis(3,5,5-trimethyl-1-hexyl) sulfosuccinate (Ag-AOT-TMH) was reduced to form silver nanoparticles at high pressure. Based on the results of two aforementioned articles, the authors suggested that the branched alky chain in isostearic acid is the key to afford the sufficient solvent-ligand interactions between silver nanoparticles and CO2. Thus, silver nanoparticles were able to be stably dispersed in CO2. However, the main drawback in their methods is the necessity of high pressure (207 bar) and cosolvent (cyclohexane) in order to improve the solvent strength of CO2 and the dispersibility of silver nanoparticles.
Instead of using scCO2 as a reaction medium, CO2-expanded liquids (CXLs) form a new class of tunable solvents for chemical syntheses. Jessop, P. G; Subramaniam, B. in an article entitled “Gas-expanded liquids” Chem. Rev. 2007, 107, 2666 describe CXLs are the mixtures where the compressed CO2 are dissolved into organic solvents accompanying volume expansion of the solutions. As compared to scCO2, CXLs are benefited by the milder operating pressure (tens of bar). Kordikowski, A.; Schenk, A. P.; VanNielen, R. M.; Peters, C. J. in an article entitled “Volume expansions and vapor-liquid equilibria of binary mixtures of a variety of polar solvents and certain near-critical solvents” J. Supercrit. Fluids 1995, 8, 205 disclose the volume expansion of organic solvents was 500% higher than by dissolving CO2 under mild pressure ranging from 40 to 70 bar. Therefore, the physicochemical properties of CXLs, including density, viscosity, solute diffusivity, and gas solubility can be adjusted easily by dissolving various amount of CO2 into organic solvents (Yin, J. Z.; Tan, C. S. Fluid Phase Equilib. 2006, 242, 111; Lin, I. H.; Tan, C. S. J. Chem. Eng. Data 2008, 53, 1886; Lin, I. H.; Tan, C. S. J. Supercrit. Fluids 2008, 46, 112, Lopez-Castillo, Z. K.; Aki, S. N. V. K.; Stadtherr, M. A.; Brennecke, J. F. Ind. Eng. Chem. Res. 2008, 47, 570; Xie, Z. Z.; Snavely, W. K.; Scurto, A. M.; Subramaniam, B. J. Chem. Eng. Data 2009, 54, 1633). Moreover, Tan's group showed that the diffusivity of solutes as well as H2 solubility could be enhanced in CXLs. (Yin, J. Z.; Tan, C. S. Fluid Phase Equilib. 2006, 242, 111; Lin, I. H.; Tan, C. S. J. Chem. Eng. Data 2008, 53, 1886; Lin, I. H.; Tan, C. S. J. Supercrit. Fluids 2008, 46, 112). Bogel-Lukasik, E.; Fonseca, I.; Bogel-Lukasik, R.; Tarasenko, Y. A.; da Ponte, M. N.; Paiva, A.; Brunner, G. in an article entitled “Phase equilibrium-driven selective hydrogenation of limonene in high-pressure carbon dioxide” reported that the hydrogenation rate of limonene in CXLs became faster compared to the pure H2 system without adding CO2. Therefore, the decrease of solution viscosity, increase of solute diffusivity, and higher H2 solubility are beneficial to improve the mass transport as well as chemical syntheses in CXLs. On the other hand, dissolving CO2 can weaken the solvating power of solvents and the precipitation of solutes is triggered in CXLs. Based on these phenomena, various methods such as gas antisolvent precipitation (GAS), precipitation with compressed antisolvent (PCA), supercritical antisolvent (SAS), solution enhanced dispersion by supercritical fluids (SEDS), and depressurization of an expanded liquid organic solution (DELOS) were adopted to precipitate fine particles composed of inorganic compounds, organic compounds, explosives, pharmaceuticals, and polymers (Jung, J.; Perrut, M. J. Supercrit. Fluids 2001, 20, 179; Shariati, A.; Peters, C. J. Curr Opin. Solid State Mat. Sci. 2003, 7, 371; Yeo, S. D.; Kiran, E. J. Supercrit. Fluids 2005, 34, 287). Recently, the deposition process of ligand-capped metal nanoparticles is applied to accomplish the uniform wide-area nanoparticle films and size-selection fractionation in CXLs (McLeod, M. C.; Anand, M.; Kitchens, C. L.; Roberts, C. B. Nano Lett. 2005, 5, 461; McLeod, M. C.; Kitchens, C. L.; Roberts, C. B. Langmuir 2005, 21, 2414; U.S. Pat. No. 7,384,879). In spite of these various merits in the field of material science and nanotechnology, however, applying CXLs as process medium to synthesize hydrophobic metal nanoparticles such as silver and palladium have not been reported to date. In order to take advantage of CXLs, an organo-soluble and fluorine-free metal precursor is also required to provide metal ions in organic medium.